High-voltage discharge lamps (lamps or bulbs) which are adapted to head lights of vehicles are driven by boosting the voltage of the car-mounted battery into a high voltage through a transformer, changing over the polarities of the high voltage through an inverter circuit, such that the lamps are turned on by an alternating current (JP-A-9-180888 and JP-A-8-321389). The transformer is provided on the primary side thereof with a switching element for controlling the primary current, the switching element being PWM-controlled (pulse width-modulated) based on the lamp voltage and on the lamp current thereby to control the electric power supplied to the lamp. Namely, a desired electric power is supplied to the lamp according to a predetermined control characteristic that specifies a relationship between the lamp voltage and the lamp current.
A lamp which is now adapted to the head light for vehicles is rated at 35 W, a lamp voltage of 85 V and a lamp current of 0.41 A. This lamp contains a trace amount of mercury. From the standpoint of environmental pollution when the lamps are disposed of, it is desired to provide a mercury-less (mercury-free) lamp. The mercury-less lamp requires a lamp voltage in a stable state which is nearly halved compared to that of the conventional counterparts. Further, the lamp voltage in the initial stage of lighting is nearly the same as that of the prior art, and is about 27 V. It further has a feature in that the light flux sharply rises in the initial stage of lighting with a slight increase in the lamp voltage. Therefore, a desired electric power is not obtained by controlling, in a customary manner, the lamp power relying upon the lamp voltage and the lamp current.
To adapt the lamp to the head light for vehicles, the light flux must be quickly increased (quickly brightened) after the lighting switch is turned on. For this purpose, the electric power larger than the rated electric power is supplied to the lamp to quicken the rise of light flux. More specifically, with the presently used 35-W lamp (bulb D2S or D2R), power of about 70 W is supplied to the lamp in the initial stage of lighting, and is then gradually decreased down to 35 W of in the stable state. This control is carried out according to a predetermined control characteristic specifying a relationship between the lamp voltage and the lamp current as shown in FIG. 13. As will be obvious from FIG. 11, the lamp voltage in the initial stage of lighting is about 27 V and is about 85 V in the stable state. The lamp power is decreased from 70 W down to 35 W as the lamp voltage is changed by 58 V from 27 V to 85 V.
Even by using the mercury-less lamp, the light flux must be quickly increased (must be quickly brightened) after the lighting switch is turned on like in the conventional control operation. For this purpose, the electric power larger than the rated power is supplied to the lamp in the initial stage of lighting to quicken the rise of light flux. More specifically, with the mercury-less 35-W lamp, the power of about 90 W must be supplied to the lamp in the initial stage of lighting, and then must be decreased down to 35 W in the stable state. The lamp voltage of the mercury-less lamp in the initial stage of turn-on is about 27 V which is nearly the same as that of the conventional lamp. However, the lamp voltage in the stable state is about 42 V which is about one-half that of the conventional lamp.
If the lamp having the above lamp voltage characteristics is controlled based on the conventional control characteristic shown in FIG. 11, the lamp power may be decreased by 55 W from 90 W down to 35 W depending upon a change of the lamp voltage by 15 V from 27 V to 42 V. Namely, with the conventional lamp, the electric power is decreased by 35 W relative to a change in the voltage of 58 V; i.e., the ratio is small. With the mercury-less lamp, on the other hand, the electric power is decreased by 55 W relative to a change in the voltage of 15 V; i.e., the ratio is large.
The lamp voltage in the initial stage of lighting is about 27 V for both the currently used lamp and the mercury-less lamp, involving a fluctuation of ± several volts. According to the presently employed control method, the fluctuation turns out to be a fluctuation in the lamp power. In the case of the mercury-less lamp, in particular, a change in the lamp voltage from the initial stage of lighting to the stable state is as small as about 15 V while the ratio is large as described above. Accordingly, the fluctuation in the lamp voltage in the initial stage of lighting seriously affects a change in the lamp power. A fluctuation in the lamp voltage until the stable state causes a large fluctuation in the light flux rise characteristics at the time of lighting, making it difficult to satisfy the standardized values specifying the light flux rise characteristics for automobiles.